Contacts and interconnections for VLSI semiconductor devices are normally formed by the deposition of a thin metal coating such as aluminum, which is then patterned to create the desired contacts and interconnections. With the increase in density of semiconductor devices, contact and interconnect resolution continues to be substantially reduced, resulting in extremely narrow metal strips. Problems have thus heretofore occurred with the use of such narrow metal strips due to electromigration. In electromigration, the atoms of the metal may be physically moved by the high current flowing therethrough, creating pillars and holes within the metal strip. This can result in failure of the metal strip due to an open circuit or due to a substantial increase in the resistance of the metal strip.
Prior techniques have been heretofore developed for making metal-to-silicon contacts through thick oxide in an effort to reduce electromigration failures. For example, in pending U.S. patent application Ser. No. 425,789, filed Sept. 28, 1982 by D. N. Anderson entitled, "Stratified Insulator Coating for Improved Contacts in VLSI Devices", a method is disclosed of forming sloping contact sidewalls by etching differing phosphorus concentrations in the oxide layer. In pending U.S. patent application Ser. No. 410,755, filed Aug. 23, 1982 by James M. McDavid, entitled "Metalization Process for Integrated Circuits", two metal depositions are utilized to provide a smoother sidewall transition and a greater thickness at steps.
Conventional techniques for control of electromigration generally employ direct alterations of the metallurgical properties of the deposited metal layer, including control of the grain size and grain boundary composition of the metal. For example, an additional metal such as copper is often used as a dopant for the aluminum. Alternatively, silicon doped aluminum interconnects are commonly used. However, such techniques for varying the metallurgical properties of the metal layer often cause a negative effect on the electrical performance of dynamic RAMs because of the copper. Also, difficulties occur because an additional metal is not usually included in processes which use chemical vapor deposition of aluminum to produce conformal metallization.
Another approach for increasing the grain size of aluminum strips has been to increase the pressure in argon sputtering of the aluminum. However, this has produced increased impurities in the aluminum and does not produce an optimum result.
A need has thus arisen for a technique for enabling the control of the metal texture, and hence, the electromigration resistance, of metal contacts and interconnects without requiring doping with a second metal or varying conventional metal deposition techniques.